The present invention relates in general to regenerative air heaters, and, in particular, to a new and useful fire or hot spot detector for regenerative air heaters.
A regenerative air heater comprises a housing containing a matrix for receiving and giving up heat. Ducts are connected to the housing for supplying heating gas to the matrix for heating the matrix and for supplying air for being heated by the matrix.
Combustible deposits can accumulate on heat transfer surfaces and elements of the matrix in a regenerative air heater. Under certain conditions, these deposits can ignite causing considerable damage to the equipment. This results in costly repairs and extensive boiler outages.
Methods are known for detecting fires or "hot spots" that lead to fires in regenerative air heaters.
U.S. Pat. No. 4,099,165 discloses the use of a thermocouple grid for fire detection in regenerative air pre-heaters. Infra-red sensors are disclosed in U.S. Pat. No. 3,730,259. Another detection apparatus is available from Fenwal, Inc. of Ashland Mass., which utilizes concentric conductors containing a eutectic salt there between, which liquifies when being exposed to a local "hot spot". The salt is conductive in its liquid state which establishes electrical contact between the conductors. This contact is used to trigger an alarm.
All known techniques for detecting fires or hot spots which lead to fires in regenerative air heaters suffer from various drawbacks.
Thermocouples must be located in the air or gas stream, some distance from the matrix or housing of the heater. This makes thermocouples insensitive as the bulk stream flow dilutes local hot spots.
Infra-red sensors utilize lenses which may become dirty, rendering the device insensitive. These sensors also depend on movement to obtain full coverage of the heat exchange area in the matrix.
Concentric conductors require the use of slip rings or other means for transmitting a signal from rotating matrix heaters.
A need thus remains for a robust hot spot detecting device which is reliable while being responsive to hot spots in the matrix regardless of their location.